Graham, have you considered that the training with the tsuba allows them to do things that otherwise would be dangerous? Therefore it allows them to do more. And it allows them to train with the weapon the bokken is ostensibly standing in for -- namely a katana.
Of course there are many who don't wish to train outside their comfort level. Or to experience more direct aspects of martial training. To generalize and call it "laziness" is simply insulting to many who damned well know the difference. I've been on the receiving end of incredibly intense matched bokken work without the tsuba in place where the extremely high level instructor (thankfully) had the control to not hurt me. But it also changed the dynamics of the encounter. Doing it again with a bokken properly fitted with the tsuba changed it dramatically allowing it to be done at full speed and intensity.
A good point made in the first paragraph. A good example of when it is useful, as long as those using live blades are responsible and quite competent.
To some experienced folk it may come across as insulting and to other experienced folk it may make them smile and be a pleasant reminder.
I too have been on the receiving end but as to using a tsuba because of it? Well, it certainly would change the dynamics but so would not being allowed to use one.
Each time the level of intensity as you put it is increased then it may be useful to use one but you won't know if you can protect your own hands or wrists until you can do the same without one.
Reliance on it therefore does lead to complacency and laziness.
Therefore I would put it in the category of a temporary aide.
Therefore I will also stand corrected on my first sentence for any so called 'offended' folk and restate it as: Reliance or using a tsuba as standard procedure leads to lazyness and complcency.